Photoinitiators used in energy-curable surface coating formulations need to have good cure speed, and particularly good surface curing activity, low odour and good solubility. Moreover, as consumers become increasingly wary of extraneous compounds in foodstuffs, in order to comply with likely future legislation, the tendency of the compounds to migrate and be extracted should also be low. Furthermore, in order for the compounds to be useful in practice, it is necessary that they should be preparable with ease and economically on a commercial scale. It is becoming increasingly difficult to meet all of these requirements.
We have now discovered a series of piperazino compounds of the aminoalkylphenone photoinitiator class which have the potential to achieve low levels of photolysis product migration and low odour from the cured print. Their strong UV chromophores in the UVB region make the aminoalkylphenones particularly useful in pigmented printing inks.
Other compounds containing piperazino groups have been suggested for use as photoinitiators in U.S. Pat. Nos. 4,321,118, 4,582,862, and EP 1357117. However, in these compounds, the piperazine ring is not attached directly to an aromatic ring, and the resulting compounds do not absorb UV radiation of the wavelengths used in commercial curing systems so efficiently.
GB 2320027 also discloses compounds similar to those of the present invention, but does not disclose compounds in which a piperazine ring is attached directly to an aromatic ring.
Thus, the present invention consists in a compound of formula (I):
where:    the substituents R1 are individually selected from C1-C10 alkyl groups and optionally substituted benzyl groups;    the substituents R2 are individually selected from alkyl groups or, together with the nitrogen atom to which they are attached, represent a nitrogen-containing heterocyclic group;    Z is selected from C6-C10 arylene groups and groups of formula —(CHR3)n—, where R3 is a hydrogen atom, a hydroxy group or a C1-C4 alkyl group, and n is a number from 0 to 6;    Y is selected from carbonyl groups and the —CH2— group;    Q is selected from the residues of mono- or poly-hydroxy compounds having from 1 to 6 hydroxy groups; and    x is a number from 1 to 6;    and esters thereof.